Apparatus for detecting internal damage in wood



June 22, 1965 P. c. TRUSSELL ETAL 3,190,111

APPARATUS FOR DETECTING INTERNAL DAMAGE IN WOOD Filed July 5 1961 INVENTORJ' PAU L C. TRUSSE LL THOMAS ANTHONY LAMBE fl-- JOHN vlTms JZWM United States Patent VAL This invention relates to a method and apparatus for detecting internal damage in wood and particularly damage caused by wood borers of the teredine type.

The teterdine borer begins life as a small, free-swimming microscopic mollusc which impinges itself on submerged wood surfaces and burrows in the wood by means of the movements of its shells. The hole formed at the surface of the wood is at first not visible to the' naked eye, and seldom exceeds one-eighth of an inch in diameter during the life of the borer. On the other hand, the borer con' tinues to increase the diameter of its burrow as it drills inward until it becomes A /2. and sometimes of an inch. When many borers attack a wood surface, as is customarily the case, the surface reveals little damage aside from miniscule perforations, whereas the internal parts of the wood become excavated by the formation of a mule tiplicity of channels densely interlaced. What appears externally to be sound wooden member is internally a perforated mass.

When the borers are alive, their presence in the wood can be notices by the projection of pairs of fine tubes extending outward from each fine pore at the surface of the wood. However, borers live only a maximum of two to three years in wood and in most cases much less because they exhaust the wood supply within their reach. This is particularly true when many borers are present in the wood. Accordingly, in most wooden structures which are not frequently inspected live borers are rarely found. More commonly the borers have lived their full life span and have died, leaving no superficial evidence of their damage except for tiny surface openings to their burrows. These openings commonly become covered by marine growth on the wood surface, and the external evidence of the attack is thus further concealed.

Making periodic inspection of wooden structures of marine-borer damage is important both for the safety of the equipment and personnel. For example, wharfs supported by wooden creosoted piling will eventually be weakened to a point of danger unless remedial measures are taken. when and what remedies are needed.

Heretofore the usual method for checking submerged wood for teredine borer attack has been to scrape or brush the surface of the wood gently (so that the creosoted coating is not damaged or perforated) and to look for the presence of the fine, surface holes. This procedure is time-consuming and inefficient. Often marine growth or particles of silt mask the borer openings. Weak sections in the wooden structuresare frequently missed. An

alternate method commonly used heretoforeis to drill holes in the wood in search of damaged areas. Each drill hole is then tightly sealed by inserting a creosoted wooden plug therein. This method is extremely laborious and haphazard. r

The present invention is concerned with a method and apparatus for detecting internaldamage in wood which.

overcomes the drawbacks in the systems employed hereto- Accurate inspection of piling will reveal Patented June 22, 1955.

fore. Ideally what is desired is a non-destructive method by which the interior of the wood can be examined for borer damage. The method should be both quick'and accurate and should be equally applicable to both untreated and chemically-impregnated wood. Further, the degree of damage should be measured mechanically and not subjectively by an operator. It is particularly important that the testing equipment be sufliciently sensitive to register that level of damage at which structural impairment commences. The equipment must be able to .operate under water; rarely is it possible to remove the Another object of the present invention is to provide a method and apparatus which enables internal damage in submerged or water-soaked wood to be measured both accurately and expeditiously.

A further object of the invention is to provide a method and apparatus which enables internal damage in sub-. merged or water-soaked wood to be measured without the necessity of any specialpreparation of the surface of the wood and in which there is no necessity to have access to the ends of the grain of the wood.

The present invention is based on the concept of detecting internal damage in submerged. or Water-soaked wood by means of sonic waves which are directed into and through the wood. The attenuation of the sound waves in their passage through the wood is then measured and from this an indication of the soundness of the wood isobtained.

In applying ultrasonic techniques to the detection of internal damage in submerged or Water-soaked wood, many difficulties are encountered. These difficulties arise mainly because of the similarity of the acoustical properties of wood and water and also because of the high attenuation of sonic vibrations in wood. The situation is aggravated by the normal variation of these properties in sound wood.

Detection by echoes or by standing waves is not pos-f siblebecause of the absence of large fiat surfaces of pronounced discontinuity at the damage. High attenuation also prevents the use of echoes with very short wave lengths. Pulse velocity methods are not sensitive to damage across the grain because of the similar velocities in water and wood in this direction and in the direction along the grain because the waves delayed by damage are obscured by faster waves transmitted along any outer layers of sound wood paralleling the damage.

A study was made of the usefulness of following the attenuation of sonic signals passing in directions across and substantially parallel to the grain of wood. Tests made in air using undamaged sepecimens and specimens which had been drilled with holes to simulate teredine burrows, gave essentially no distinction between the two specimens. Further, when water was used to improve the contact between the transmitting transducer and the wood and the receiving transducer and the wood, damage still could not be detected in wood intersected by air passages.)

After exhaustive tests made on wood specimens in before other components obscure the reception.

7 ing water.

water it was determined that in contrast to the results obtained where sound was directed-along and across the grain of Wood in air, there is a difference in the attenuation of sound wavesdirected substantially along the 7 grain in the undamaged and borer-damaged specimens tested under water enabling varying degrees of damage to be revealed and to some extent the amount of damage to be measured. was a definite attenuation of the-high velocity com ponents of the waves in the damaged specimens. It was further determined that thesewave components could be generated and detected after passage along through the grain of the specimen 'b'y transducers positioned adjacent a side surface of the specimen rather than at the ends of the specimen. This arrangement makes it pos sible to search for internal damage along the complete length of a specimen such ,as a. piling, without requiring access to the ends of the piling.

It was thus discovered that sonic waves and specifically the high velocity components. thereof travel with'less, loss of strength along the grain'of sound wood than they do along the grain of wood which contains channels a ofwater or live teredine specimens. It was found that a pulse of sonic waves applied to the surface of the wood' generates a component .which travels substantially in the direction of the 'grainin sound wood at essentially In particular it was found that there a 4 V FIGURE 2 is a sectional view on the line 22 of FIGURE l, and

' FIGURE 3 is a schematic diagram showing the .layout of the apparatus and associated circuitry. Referring to FIGURES 1 and 2 of the drawings, the

apparatus comprises a shaft provided adjacent eachend with channel-shapedbrackets 12 and 14. The shaft 10 is provided with-a covering 16 of a suitable sound insulating materiad, such as rubber. The brackets 12 and 14 form mounts for clamping assemblies generally in-' dicated at lfi and Zflwhich enable the apparatus to be clamped in position around a specimen such as a section of wood piling 22 which is to be tested. Theseclamping assemblies 18 and 20 are identical and it will thus be necessary to-describeonly one assembly 18 in' detail. Clamping assembly 18 includes a pair of channel-shaped arm members 24 and 26, each pivotally connected at one the velocity of a compressional wave in that direction and.

that the presence of internal damage 'in the submerged or water-soaked wood attenuates the before-mentioned component of the wave' to an extent that increases with borers are mainly water in composition. It was also discovered that the before-mentioned component of 'the sonic waves has the highest velocity and hence shortest time delay of any component of the waveaction with the result that changes in attenuation'can' be determined The results also showed that the sonic Waves and specificially the high-velocity components thereof travel much faster end adjacent opposite ends of bracket 12. 'A spring 27 connected at opposite .ends to arms 24 and 26 respectively, is efiective to bias the arms about their pivots towardsteach, other. Links 28 and 30 pivotally attached i atone end to arms 24 and 26 respectively are pivotally interconhectedattheir other ends by a pin '32 which is slidably engaged in'a slot 34 formed in an extension 36 of bracket 12. The. links 28 and 30 are effective to guide the movement of arms 24 and 26 and the pin 32 and slot 34 coupling serves to limit the movementof these; arms relative to each other. Angle members 38 and 40 are secured to and project normally from the free ends'of arms 24 and 26 and are provided withU-shaped brackets 42 and 44 respectively. Rollers 46 and 48' are 7 rotat'ably mounted on axles '50 and 52 journalled in brackets 42 and 44 respectively. Further rollers 54and 5 6 rotatably mounted on axles 58 and 60 are journalled in U-shapedbrackets 62 and 64 securedto shaft 10 in;

termediate the clamping assemblies 18. and 20.1 The clamping assemblies 13 and'20 are adapted to fit around the, piling 22 with the rollers 46 and 48 engaging the periphery thereof and urging the piling against the backing rollers;54 and 56 under the action of the springs 27.

'A pair of brackets 66 and 68 are slidably mounted on i shaft 10 for movement therealong between thejrollers in sound wood than sonic waves travel in the surroundv The present method of determining internal damage in wood comprises the steps of directing sound 'waves into and. substantially along. the grain of said wood and measuring the attenuation'of the sound waves occurring i in said wood, the degree of attenuation of said sound waves providing an indication of the soundness of the wood. a

Apparatus for carrying out the method of this invention comprises. means for generating a pulse. of some waves, means for directing said waves into. and substantially along the grain of the wood, means spaced from said generating means longitudinally in'the direction of the wood grain for receiving the sound waves trans ;mitted through the wood andmeans for measuring the" attenuationof the sound waves transmitted through the wood thereby to provide an indication of the soundness of the wood. V

Other features which] may 'be included in accordance with the invention will be described hereinafter. and re f l'red to in the PP claims" I i The invention will now be more'particularly described in connection'with the accompanying drawings which 5 4' and 56. Bolts 70 and 72 enable the brackets 66 and 68 respectively to be locked in selected positions alongf the shaft 10..

Y A sound transmitter. 74 having an accoustical reflector. 75 is pivotally mounted on bracket 66 and a sound re-' ceiver 76 having an acoustical reflector 77 is pivotally mounted on bracket-68.- The use of hornreflectors 'se-' cures maximum transfer of sonic energy of the waves fromthe transmitter into and along the wood and from the wood to the receiver.- The transmitter 74 comprises a pulse generator adapted to produce short electrical pulses which are applied to a transducer 78 causing the latter to resonate at its natural frequency of oscillation. The transmitting transducer 78 is veifective to convert the energy of the electn'cal pul'ses developed bysaid pulse 1 generator into short bursts of oscillation as, sonic waves.

Preferably, the transmitting transducer is of the magnetostrictive type producing waves at a frequency of 30 kilo cycles persecond. Other: types of sound transducers.

' .such as piezo-electric or'electromagnetic may however 7 other frequencies maybe used. 'It-has been found that the upper limit of thefrequency of the sound wavesiuse- 1 ful fordetermining damage iriwood is kilocycles per second. Waves' of :high frequency'attenuate rapid:

showby way of example oneembodiment of the invention, and in which: 7 t i FIGURE. lis a' view inside elevation of one form of apparatus according to the invention to a section of wood piling, V r

shown attached be used if desired. It. isto be noted, however, that al: though it is preferred to u'sesonic waves with a he quency of '30 kilocycles per second, sonic waves with 1y even in sound wood. Waves with afrequency of less thangl kilocycle per second 'do not transmit welLfrom the transmitter to the wood or. from the wood to the receiver. Thepracticalrange of .wave lengths has been; found tolie between 10 and 8 0 kilocycles per second.

a The. transmitting transducer isenergized from a suit able pulse generating circuit. As shown in FIGURE 3, the pulse generator may comprise a condenser C1 which is charged from a suitable power supply B through a resistance R. The condenser C1 is connected across the terminals of the transmitting transducer through contacts 90. The contacts 90 are periodically closed through a cam 92 mounted on a shaft 93 driven by a suitable motor 94 adapted to close the contacts 90 approximately 5 times per second. With'the contacts closed,'the condenser Cl is discharged across the transmitting transducer and in this manner pulses of electrical energy are supplied to the transmitting transducer at the rate of ap-' proximately '5 pulses per second.

The receiver includes a transducer 79, which may be of the magneto-strictive, piezo-electric or electromagnetic type, provided with a reflector 77 and adapted to convert sonic waves impinging thereon into electrical waves. The electrical waves from the receiver are applied through a step-up transformer 98 to a cathode ray oscilloscope 100 from which the attenuation of the signal transmitted through the piling can be measured. The

sweep circuit connected to the horizontal deflector terminals of the oscilloscope is connected through a capacitor C2 to the pulse generator and its operation is initiated.

by and synchronized with the electrical pulses generated by the pulse generator. time base is required for identifying the high velocity wave component for attenuation measurement.

For best sensitivity, the transmitter and receiver are directed towards each other preferably at an angle of 20 plus or minus 5 degrees to the perpendicular of the surface of the piling, although detection of damage can be obtained over the approximate range from 0 to 40 degrees to the perpendicular. The transmitter and receiver are maintained a constant distance from the surface of the piling by means of the backing rollers 54 and 56. Preferably, the transmitter reflector 75 and the receiver reflector 77 are positioned at 'a distance of up to approximately 15 centimeters from the surface of the piling, although greater distances could be used. It has been found that with the transmitter and receiver arranged in this manner, the sound waves are transmitted substantially along the grain of the wood under water with essentially no difference in the rate or strength of travel. This eliminates the variations in sound transmission associatedwith making transmitter-to-wood and wood-to-receiver contacts and enables more standardized testing conditions to be achieved. Furthermore, it avoids the complications which arise when barnacles, tunicates and other small marine growth are on the surface of the wood being tested. With the use of transmitter and receiver reflectors spaced a short distance from the wood surface, the small forms of marine growth have little effect and do not need to be removed to obtain reliable results on the internal condition of the wood.

In the operation of the device, the'transmitter and receiver are secured in position on shaft at a desired special separation of, for example, 1 meter. The distance separating the transmitter and receiver is of some importance and where the transmitting frequency is 30 kilocycles, the minimum distance for detecting damage.

under practical conditions has been about 80 centimeters. A longer distance is preferable because it gives a greater diflerence in the strength of the signal between damaged and undamaged wood. The 'extent to which the transmitter and the receiver can be moved apart, depends upon the attenuation of the sound by the wood. With 30 kilocycle equipment distances over 140 centimeters resequently may be more sensitive to damage closer to the centre and to the opposite side of the pile.

The synchronized horizontal With the transmitter and receiver set up in this manner,

pulsed sonic waves are directed from the transmitter into and along the grain of the piling. After passing in and down the piling for a distance of 1 meter, the signal is picked up by the receiver which, in turn, passes it along to the vertical amplifier input terminals of the cathode ray oscilloscope on which the strength of the signal and its time of passage for the fixed distance in the wood are measured. The sensitivity of the vertical amplifier of the oscilloscope is adjusted for 20 milli-volts per centi meter. The sound wood permits the reception of sonic signals having an amplitude of 30 to 60 millimeters for the initial 200 microseconds of the received compressional wave, having a velocity of approximately 5000 meters per second, whereas seriously damaged wood gives signals with amplitudes of only 0-5 millimeters.

Greater sensitivity to borderline damage tained by calibrating the apparatus prior to testing a speciment. The calibration is effected with a sample of sound wood of the same species and general shape as the specimen to be inspected such, for example, as a region of the specimen that is known to be sound for other reasons. The calibration is effected under the same conditions as those to be encountered in the tests on the specimen to be examined. With the apparatus mounted on the sample, the attenuation of signals transmitted through the sample with the specific time delay are then measured.

Thereafter, when the apparatus is connected to the speci In testing piling, the apparatus maybe raised and low-,

ered on the piling by means of a cable connected at one end to shaft 10 and connected at its other end to a suitable hoist mechanism, although preferably a diver is em ployed to move the apparatus. In the latter case, the diver can raise, lower and rotate the unit in a horizontal direction around the piling. The diver also facilitates the manipulation of the unit around obstructions, such as brace piling.

As shown, the transmitter and receiver are located on the same surface of the specimen being tested. This arrangement is the most convenient one and provides greater sensitivity torlocalized damaged. 7 However, it has,

been found that detection is possible with the transmitter and receiver angularly displaced up to degrees from one another.

What we claim as our invention is:

1. Apparatus for detecting internal damage in a wood specimen submerged in water and caused by the action of wood borers and resulting in the creation of moistureladen pockets within the wood comprising an elongated support member adapted to be positioned adjacent to and spaced from a surface of the specimen and positioned parallel to the grain of said specimen, a sound transmitter mounted on said support member and positioned to direct pulses of sound wave oscillations into the adjacent surface of the wood so as to form a series of compressional waves whose front travels parallel to the grain of the specimen, a sound receiver mounted on said support member and spaced from said transmitter longitudinally of said support member, said receiver adapted to detect the sound waves radiating from the surface of the wood after travelling along the grain of the specimen, the degree of attenuation of said sound waves providing an indication of the soundness of said specimen.

2. Apparatus for detecting internal damage in a wood specimen submerged in water and caused by the action ofrwood borers and resulting in the creation of moisture laden pockets within the Wood comprising an elongated support member, clamping means mounted on said support member adjacent each end thereof, said clamping means adapted for releasable engagement with the specimen to secure said support member to said specimen in ear-15s ob- 7.. spaced relationship and substantially parallel to the grain thereof, a sound transmittermounted onsaid support member and positioned to direct pulses of sound wave oscillations into the adjacent surface of the Wood so as to form a series of compressional waves whose front travels parallel to the grain of the specimen, a sound receiver mounted on-said support member and spaced from said transmitter longitudinally of said support member, said receiver adapted to detect the sound waves radiating from the surface of the wood after travelling along the grain ofthe specimen, the degree of attenuation of said sound waves providing an indication of the soundness of said specimen. r

3. Apparatus as claimed in claim Z wherein each of said clamping means comprises a bracket secured to said 7 support member adjacent an end thereof, a pair'of arms pivotallyconnected to saidbr'acket adjacent one of their ends and-having rollers rotatably mounted adjacent their other ends, and means normally urging said arms towards 7 each other and operable to urge-said rollers into engageso as to form a series of compressional'waves whose front travels parallel to the grain of the specimen, receiving means for detecting said sound waves'after transmission thereof through the specimen, said receiving means mounted on said support member and spaced from said generating means, and means connected to saidreceiving means for indicating and measuring the attenuation of the sound waves radiating from the surface of the wood. after travelling along the grain of the specimen, the degree of attenuation of said sound waves providing an indication of the soundness of the specimen.

5. Apparatus for detecting internal damage in a Wood specimen submerged-in water and caused by the action of wood borers and resulting in the creation of moistureladen pockets within the wood comprising an'elongated support member, means for releasably clamping said support member on to said specimen spaced therefrom and extending substantially parallel to the grain thereof, means forgenerating' pulses of sound wave oscillations, said generating means mounted on said support member and spaced'from a surface of said specimen, means coupled to said generating means for directing said sound waves into the adjacent surface of the wood so as to form a series of compressionalwaves whose front travels parallel to-the grain of thespecimen, receiving means for detecting said sound wavesradiating from the surface of the wood after travelling along the grain of the speci. men, said receiving means spaced from said surface and spaced from said generating means in a direction'longitudinally of said support member, and means connected I to said receiving means for'indicating' and measuring-the attenuation of the sound waves transmitted through the specimen; the degree ofattenuation of .said sound waves providing an indication of the soundness of the specimen.

, 6 Apparatus for detectinginternal damage in wood" submerged in water comprising means generating pulses of sound waves, means directing said soundwaves into the woodrthrough a surface thereof that parallels the grain of said Wood and so as to form a series of'compressional waves Whose front travels parallel to the grain of said wood, means spaced from said generating means in adirection lengthwise of the grain of the wood for" detecting the sound waves radiating from the surface of the Wood after travelling along the grain of said wood,:

.ness ofsaid wood.

ing' and measuring the attenuation of said sound waves transmitted through the 'wood, the degree of attenuation,

of said sound waves providing an indication of the soundof wood borers and resulting in the creation of moistureladen pockets within'the wood comprising a sound trans-. mitter, a sound receiver, means retaining the transmitter and the receiver spaced from each other and spacedfrom a surface of the specimen that parallels the grain of said specimen, said, sound transmitter being positioned to direct sound wavesinto saidsurface of the wood so as to form compressionalwaves whose front travels parallel to the grain of. the specimen, and said sound receiver being positioned to detect the sound Wavesradiating from the surface of the wood after travelling along'the grain of the specimen, the degree of attenuation of said sound waves providing an indication of the soundness of said specimen.

8. Apparatus. as claimed in claim' 7 in which the said sound waves have'a frequency in the range 1 to 100' 'kilocyclesjper second. 1

9. Apparatus as claimed in claim 7 in which the transmitter and the receiver are directed substantially towards each other and'inclined at an angle in the range 0 to 35 degrees to the normal ofsaid surface of the wood.

: 10; Apparatus. for detecting internal damage in a wood specimen submergedin water and caused by the action of wood borers and resulting in the creation of moisture laden pocketswithin the wood comprising a sound transmitter, a sound receiver, means retaining the transmitter and the receiver spaced from each other and spaced from a surface of the specimen that .parallels the grain of said specimen, said sound transmitter being positioned to dii rect pulses of sound waves into said surface" of the Wood so as to form' a series of compressional waves whose front travels parallel to the grain of. the specimen, said sound waves having a frequency in the range of 1 to 100 kilo-j cycles per second with a pulse repetition rate of from '2 to 20 per second, and said sound receiver being posi- V tioned to detectthe sound waves radiating from the surface of the wood after travelling along the grain of the specimen, the degree of attenuation of said sound waves providing'an indication of the soundness of said specimen. 11. Apparatus for detectinginternal damage in a wood specimen submerged in water and caused by the action of wood borers and resulting in the creation of moistureladen pockets within the wood comprising a sound transmitter, asound receiver, means retaining the transmitter and the receiver spaced approximately one meter from grain, said'sound transmitter being positioned to direct; sound waves into'saidsu'rface of the Wood was to form.

' grain of the specimen, and said sound receiver being poeach other in a directionlengthwise of the grain of 'the wood and each spaced a'distance up'to approximately 15 centimeters from. a surface, of the wood that parallels said compressional Waves whose front travels parallel to the sitioned to detect the sound waves radiating from the surface of the Wood aftertravelling along the grain of the specimen, the degreeof attenuation of said sound waves" providing an indication of the soundness of said specimen; 12. Apparatus for detecting internal damage in wood 7 submerged in water, comprising means generating sound waves,'means directing said sound waves into the wood through a surface thereof that parallels the grain of said Wood and so as to form compressional waves whose front travels parallel to the 'grainof said wood, means spaced from said generating means in a direction lengthwise of the grain of the wood-for detecting the, sound waves.

radiating from the surface' of the wood after travelling along the grain of said wood, and means connected: to

said detecting means for indicating and measuring the at tenuation of said sound waves; transmitted through the Wood, the degree of attenuation of said sound Waves pro- 7 OTHER REFERENCES viding an indication of the soundness of said wood. IMCMaSter: Non Destructive Testing Handbook, vol.

11, pages 48.1, 49.10, 43.1-43.21, Ronald Press 03., N.Y.

References Citefi by the Examiner V 1959 FOREIGN PATENTS 5 494 971 8/53 Canada RICHARD C. QUEISSER, Primary Examiner. 

7. APPARATUS FOR DETECTING INTERNAL DAMAGE IN A WOOD SPECIMEN SUBMERGED IN WATER AND CAUSED BY THE ACTION OF WOOD BORERS AND RESULTING IN THE CREATION OF MOISTURELADEN POCKETS WITHIN THE WOOD COMPRISING A SOUND TRANSMITTER, A SOUND RECEIVER, MEANS RETAINING THE TRANSMITTER AND THE RECEIVER SPACED FROM EACH OTHER AND SPACED FROM A SURFACE OF THE SPECIMEN THAT PARALLELS THE GRAIN OF SAID SPECIMEN, SAID SOUND TRANSMITTER BEING POSITIONED TO DIRECT SOUND WAVES INTO SAID SURFACE OF THE WOOD SO AS TO FROM COMPRESSIONAL WAVES WHOSE FRONT TRAVELS PARALLEL TO THE GRAIN OF THE SPECIMEN, AND SAID SOUND RECEIVER BEING POSITIONED TO DETECT THE SOUND WAVES RADIATING FROM THE SURFACE OF THE WOOD AFTER TRAVELLING ALONG THE GRAIN OF THE SPECIMEN, THE DEGREE OF ATTENUATION OF SAID SOUND WAVES PROVIDING AN INDICATION OF THE SOUNDNESS OF SAID SPECIMEN. 